Photocatalytic electrode for water splitting and water splitting device

ABSTRACT

An object of the present invention is to provide a photocatalytic electrode for water splitting and a water splitting device excellent in the onset potential. The water splitting device of the present invention is a water splitting device which generates gases from a photocatalytic electrode for hydrogen generation and a photocatalytic electrode for oxygen generation by irradiating the photocatalytic electrode for hydrogen generation and the photocatalytic electrode for oxygen generation with light, and includes a bath to be filled with an electrolytic aqueous solution and the photocatalytic electrode for hydrogen generation and the photocatalytic electrode for oxygen generation each disposed in the bath. The photocatalytic electrode for hydrogen generation has a p-type semiconductor layer, an n-type semiconductor layer provided on the p-type semiconductor layer, and a co-catalyst provided on the n-type semiconductor layer. The p-type semiconductor layer is a semiconductor layer containing a CIGS compound semiconductor containing Cu, In, Ga, and Se, and a molar ratio of Ga to a total molar amount of Ga and In in the CIGS compound semiconductor is 0.4 to 0.8.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of PCT International Application No. PCT/JP2019/007233 filed on Feb. 26, 2019, which claims priority under 35 U.S.C. § 119(a) to Japanese Patent Application No. 2018-040134 filed on Mar. 6, 2018. The above application is hereby expressly incorporated by reference, in its entirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a photocatalytic electrode for water splitting and a water splitting device.

2. Description of the Related Art

It is known that a CIGS compound formed of an alloy of Cu, In, Ga, and Se is used as a light absorbing layer of a solar battery, due to the fact that it has excellent sunlight conversion efficiency. For example, Phys. Status Solidi RRL 10, No. 8, 583-586 (2016)/DOI 10.1002/pssr. 201600199 discloses that in a case where the molar ratio of Ga to the total amount of Ga and In is 0.3 as a CIGS compound used for a light absorbing layer of a solar battery, the sunlight conversion efficiency is particularly excellent.

In recent years, in addition to the above-described photoelectric conversion device such as a solar battery that converts light into electricity, as a method of utilizing solar energy, attention has been focused on technology for producing hydrogen and oxygen by decomposing water using a photocatalyst.

SUMMARY OF THE INVENTION

In order to increase the amount of generated gas, a photocatalytic electrode for water splitting is required to have a high current value in a case of being applied to a water splitting device. In this case, the potential (onset potential) from which the current value can be extracted is also important.

A water splitting device that uses a system in which water can be decomposed under the irradiation with visible light to generate both hydrogen and oxygen (so-called “Z scheme”) by using a photocatalytic electrode for hydrogen generation and a photocatalytic electrode for oxygen generation is known.

In a case of focusing on a water splitting device having two photocatalytic electrodes using the Z scheme, the ability to extract a current at a potential (onset potential) having a voltage of about half or more of the voltage (about 1.23 V) required for water decomposition in one photocatalytic electrode is an index for the performance of a water splitting device.

From this viewpoint, the present inventors applied the CIGS compound semiconductor used for a solar battery, which is described in the above literature, to a photocatalytic electrode for water splitting, but the onset potential was insufficient and the performance required for a water splitting device was not satisfied.

Therefore, an object of the present invention is to provide a photocatalytic electrode for water splitting and a water splitting device excellent in the onset potential.

As a result of performing intensive studies on the above problems, the present inventors have found that in a case where a photocatalytic electrode for water splitting that has a semiconductor layer containing a CIGS compound semiconductor is applied to a water splitting device, the onset potential is excellent in a case where the molar ratio of Ga to the total amount of Ga and In in the CIGS compound semiconductor is within a predetermined range, and the present invention has been achieved.

In addition, the present inventors have found that, in a water splitting device having a photocatalytic electrode for water splitting having a p-type semiconductor layer and an n-type semiconductor layer, the onset potential of the photocatalytic electrode for water splitting is excellent in a case where the band offset, which is the difference between the potential at the lower end of the conduction band of the p-type semiconductor layer and the potential at the lower end of the conduction band of the n-type semiconductor layer, is equal to or less than a predetermined value.

That is, the present inventors have found that the above-described problems can be solved by the following configurations.

[1] A water splitting device which generates gases from a photocatalytic electrode for hydrogen generation and a photocatalytic electrode for oxygen generation by irradiating the photocatalytic electrode for hydrogen generation and the photocatalytic electrode for oxygen generation with light, the water splitting device comprising:

a bath to be filled with an electrolytic aqueous solution; and

the photocatalytic electrode for hydrogen generation and the photocatalytic electrode for oxygen generation each disposed in the bath,

in which the photocatalytic electrode for hydrogen generation has a p-type semiconductor layer, an n-type semiconductor layer provided on the p-type semiconductor layer, and a co-catalyst provided on the n-type semiconductor layer,

the p-type semiconductor layer is a semiconductor layer containing a CIGS compound semiconductor containing Cu, In, Ga, and Se, and

a molar ratio of Ga to a total molar amount of Ga and In in the CIGS compound semiconductor is 0.4 to 0.8.

[2] The water splitting device according to [1], in which the n-type semiconductor layer contains CdS.

[3] The water splitting device according to [1] or [2], further comprising a metal layer provided between the n-type semiconductor layer and the co-catalyst.

[4] The water splitting device according to any one of [1] to [3], in which the molar ratio of Ga to the total molar amount of Ga and In in the CIGS compound semiconductor is 0.5 to 0.7.

[5] A water splitting device which generates gases from a photocatalytic electrode for hydrogen generation and a photocatalytic electrode for oxygen generation by irradiating the photocatalytic electrode for hydrogen generation and the photocatalytic electrode for oxygen generation with light, the water splitting device comprising:

a bath to be filled with an electrolytic aqueous solution; and

the photocatalytic electrode for hydrogen generation and the photocatalytic electrode for oxygen generation each disposed in the bath,

in which the photocatalytic electrode for hydrogen generation has a p-type semiconductor layer, an n-type semiconductor layer provided on the p-type semiconductor layer, and a co-catalyst provided on the n-type semiconductor layer, and

a band offset ΔE which is a difference between a potential p-CBM at a lower end of a conduction band of the p-type semiconductor layer and a potential n-CBM at a lower end of a conduction band of the n-type semiconductor layer satisfies the following relationship,

ΔE=(n-CBM)−(p-CBM)≤0.1 [eV].

[6] A photocatalytic electrode for water splitting, the electrode comprising: a semiconductor layer containing a CIGS compound semiconductor containing Cu, In, Ga, and Se,

in which a molar ratio of Ga to a total molar amount of Ga and In in the CIGS compound semiconductor is 0.4 to 0.8.

As described will be later, the present invention can provide a photocatalytic electrode for water splitting and a water splitting device excellent in the onset potential.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing a photocatalytic electrode for water splitting according to the one embodiment of the present invention.

FIG. 2 is a schematic sectional view showing a photocatalytic electrode for water splitting according to the one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described.

A numerical value range represented using “to” in the present invention means a range including the numerical values described before and after “to” as the lower limit and the upper limit respectively.

In the present invention, visible light is light having a wavelength visible to the human eye among electromagnetic waves, and specifically, light in a wavelength range of 380 to 780 nm.

In the present specification, “onset potential is excellent” means that the value of the onset potential is 0.6 V (vs. RHE) or more. Here, RHE is an abbreviation for reversible hydrogen electrode. In a case of being used as a photocatalytic electrode, it is preferable that more current is obtained at 0.6 V (vs. RHE).

Hereinafter, a water splitting device of the embodiment of the present invention will be described in detail for each embodiment with reference to the drawings.

First Embodiment

In one embodiment of a water splitting device of the present invention, the water splitting device generates gases from a photocatalytic electrode for hydrogen generation and a photocatalytic electrode for oxygen generation by irradiating the photocatalytic electrode for hydrogen generation and the photocatalytic electrode for oxygen generation with light, and includes a bath to be filled with an electrolytic aqueous solution and the photocatalytic electrode for hydrogen generation and the photocatalytic electrode for oxygen generation each disposed in the bath, in which the photocatalytic electrode for hydrogen generation has a p-type semiconductor layer, an n-type semiconductor layer provided on the p-type semiconductor layer, and a co-catalyst provided on the n-type semiconductor layer, the p-type semiconductor layer is a semiconductor layer containing a CIGS compound semiconductor containing Cu, In, Ga, and Se, and a molar ratio of Ga to a total molar amount of Ga and In in the CIGS compound semiconductor is 0.4 to 0.8. In this specification, the molar ratio of Ga to the total amount of Ga and In in the CIGS compound semiconductor may be simply abbreviated as “Ga ratio”.

The photocatalytic electrode (specifically, the photocatalytic electrode for hydrogen generation) in the water splitting device of the first embodiment shows an excellent onset potential. Although the details of the reasons for this property have not been clarified, it is assumed that the reasons are generally as follows.

That is, in a case where a CIGS compound semiconductor having a Ga ratio of 0.3, which is an optimal composition as a light absorbing layer constituting a solar battery, is applied to the photocatalytic electrode, the offset potential between the conduction band edge (the bottom portion of the conduction band) of the CIGS compound semiconductor and the conduction band edge (the bottom portion of the conduction band) of a layer adjacent thereto (for example, an n-type semiconductor layer described) is high, which is considered to be a barrier to inhibit carrier transport. Therefore, the conduction band edge (the bottom portion of the conduction band) of the CIGS compound semiconductor is preferably equal to or shallower than the conduction band edge (the bottom portion of the conduction band) of a layer adjacent thereto (for example, an n-type semiconductor layer described later). Specifically, the band offset ΔE (see the following expression) which is a difference between a potential p-CBM at the lower end of the conduction band of a p-type semiconductor layer and a potential n-CBM at the lower end of the conduction band of an n-type semiconductor layer is preferably 0.1 eV or less and more preferably 0 eV or less. In addition, the lower limit of ΔE is preferably −0.5 eV or more.

ΔE=(n-CBM)−(p-CBM)

In a case where the Ga ratio is 0.4 or more, the conduction band edge (the bottom portion of the conduction band) of the CIGS compound semiconductor becomes shallow, the offset potential between the CIGS compound semiconductor and a layer adjacent thereto becomes low, and thus it is presumed that an excellent onset potential is exhibited.

In the present invention, p-CBM and n-CBM are obtained by adding the potential at the upper end of the valence band, which is determined using an atmospheric photoelectron spectrometer (product name “AC-3”, manufactured by Riken Keiki Co., Ltd.), with the value of the band gap, which is determined from an ultraviolet-visible spectrophotometer (product name “V-770”, manufactured by JASCO Corporation).

In the present invention, the value of the potential at the lower end of the conduction band is a value in a case where the axis is set to the minus side with respect to the vacuum level as a reference (0 eV).

Hereinafter, the configuration of the water splitting device of the first embodiment will be described with reference to the drawings.

FIG. 1 is a side view schematically illustrating a water splitting device 1 which is one example of a water splitting device of a first embodiment of the present invention. The water splitting device 1 is a device that generates gases from an anode electrode 10 (photocatalytic electrode for oxygen generation) and a cathode electrode 20 (photocatalytic electrode for hydrogen generation) by irradiation with light L. Specifically, water is decomposed by the light L, oxygen is generated from the anode electrode 10, and hydrogen is generated from the cathode electrode 20.

As shown in FIG. 1, a water splitting device 1 includes: a bath 40 filled with an electrolytic aqueous solution S; the anode electrode 10 and the cathode electrode 20 each disposed inside the bath 40; and a diaphragm 30 disposed between the anode electrode 10 and the cathode electrode 20 and inside the bath 40. The anode electrode 10, the diaphragm 30, and the cathode electrode 20 are arranged in this order along the direction intersecting the traveling direction of the light L.

As the light L for irradiation, visible light such as sunlight, ultraviolet light, infrared light, or the like can be used, and among these, sunlight whose amount is infinite is preferable.

<Bath>

The inside of the bath 40 is divided by the diaphragm 30 into an anode electrode chamber 42 in which the anode electrode 10 is disposed and a cathode electrode chamber 44 in which the cathode electrode 20 is disposed.

Although not limited thereto, the bath 40 is disposed to be inclined so that the amount of incident light per unit area with respect to the anode electrode 10 and the cathode electrode 20 increases. The bath 40 is sealed so that the electrolytic aqueous solution S does not flow out in a state where the bath 40 is inclined.

Regarding a specific example of the material that constitutes the bath 40, a material having excellent corrosion resistance (particularly, alkali resistance) is preferable, and examples thereof include polyacrylate, polymethacrylate, polycarbonate, polypropylene, polyethylene, polystyrene, and glass.

(Electrolytic aqueous solution)

As shown in FIG. 1, the inside of the bath 40 is filled with the electrolytic aqueous solution S, and the entirety of the anode electrode 10, the cathode electrode 20, and the diaphragm 30 is immersed in the electrolytic aqueous solution S.

The electrolytic aqueous solution S is a solution in which an electrolyte is dissolved in water. Specific examples of the electrolyte include sulfuric acid, sodium sulfate, potassium hydroxide, potassium phosphate, and boric acid.

The pH of the electrolytic aqueous solution S is preferably 6 to 11. In a case where the pH of the electrolytic aqueous solution S is within the above range, there is an advantage that the solution can be handled safely. The pH of the electrolytic aqueous solution S can be measured using a known pH meter, and the measurement temperature is 25° C.

The concentration of the electrolyte in the electrolytic aqueous solution S is not particularly limited, but it is preferable that the pH of the electrolytic aqueous solution S is adjusted to be in the above-described range.

<Anode electrode>

The anode electrode 10 is disposed in the anode electrode chamber 42.

The anode electrode 10 has a first substrate 12; a first conductive layer 14 disposed on the first substrate 12; and a first photocatalyst layer 16 disposed on the first conductive layer 14. The anode electrode 10 is disposed in the bath 40 so that the first photocatalyst layer 16, the first conductive layer 14, and the first substrate 12 are arranged in this order from the side irradiated with the light L.

In the example of FIG. 1, the anode electrode 10 has a flat plate shape; however, the shape is not limited to this. The anode electrode 10 may be in a punched metal form, a mesh form, or a lattice form, or the anode electrode 10 may be a porous body having penetrating pores.

The anode electrode 10 is electrically connected to the cathode electrode 20 by a conducting wire 50. FIG. 1 shows an example in which the anode electrode 10 and the cathode electrode 20 are connected by the conducting wire 50; however, the mode of connection is not particularly limited as long as the electrodes are electrically connected.

The thickness of the anode electrode 10 is preferably 0.1 to 5 mm, and more preferably 0.5 to 2 mm.

(First substrate)

A first substrate 12 is a layer that supports the first conductive layer 14 and the first photocatalyst layer 16.

Specific examples of the material constituting the first substrate 12 include metals, organic compounds (for example, polyacrylate and polymethacrylate), and inorganic compounds (for example, metal oxides such as SrTiO₃, glass, and ceramics).

The thickness of the first substrate 12 is preferably 0.1 to 5 mm, and more preferably 0.5 to 2 mm.

(First conductive layer)

Since the anode electrode 10 has the first conductive layer 14, electrons generated by the incidence of light L on the anode electrode 10 move to the second conductive layer 24 (will be described later) of the cathode electrode 20 via the conducting wire 50.

Specific examples of the material forming the first conductive layer 14 include metals (for example, Sn, Ti, Ta, and Au), SrRuO₃, ITO (indium tin oxide), and zinc oxide-based transparent conductive materials (such as Al:ZnO, In:ZnO, and Ga:ZnO). The notation of “metal atom:metal oxide” such as Al:ZnO means that a portion of the metal (Zn in the case of Al:ZnO) constituting the metal oxide has been substituted with metal atom (Al in the case of Al:ZnO).

The thickness of the first conductive layer 14 is preferably 50 nm to 1 μm, and more preferably 100 to 500 nm.

A method for forming the first conductive layer 14 is not particularly limited, and includes, for example, a vapor deposition method (for example, a chemical vapor deposition method and a sputtering method).

(First photocatalyst layer)

In a case where the anode electrode 10 is irradiated with the light L, electrons generated in the first photocatalyst layer 16 move to the first conductive layer 14. On the other hand, as the holes (positive holes) generated in the first photocatalyst layer 16 react with water, oxygen is generated from the anode electrode 10.

The thickness of the first photocatalyst layer 16 is preferably from 100 nm to 10 μm, and more preferably from 300 nm to 2 μm.

Examples of the material capable of constituting the first photocatalyst layer 16 can include oxides such as Bi₂WO₆, BiVO₄, BiYWO₆, In₂O₃(ZnO)₃, InTaO₄, and InTaO₄:Ni (where the expression “compound:M” indicates that an optical semiconductor is doped with M. The same applies hereinafter.), TiO₂:Ni, TiO₂:Ru, TiO₂Rh, TiO₂:Ni/Ta (the expression “compound:M1M2” indicates that an optical semiconductor is co-doped with M1 and M2. The same applies hereinafter.), TiO₂:Ni/Nb, TiO₂:Cr/Sb, TiO₂:Ni/Sb, TiO₂:Sb/Cu, TiO₂:Rh/Sb, TiO₂:Rh/Ta, Ti₂:Rh/Nb, SrTiO₃:Ni/Ta, SrTiO₃:Ni/Nb, SrTiO₃:Cr, SrTiO₃:Cr/Sb, SrTiO₃:Cr/Ta, SrTiO₃:Cr/Nb, SrTiO₃:Cr/W, SrTiO₃:Mn, SrTiO₃:Ru, SrTiO₃:Rh, SrTiO₃:Rh/Sb, SrTiO₃:Ir, CaTiO₃:Rh, La₂Ti₂O₇:Cr, La₂Ti₂O₇:Cr/Sb, La₂Ti₂O₇:Fe, PbMoO₄:Cr, RbPb₂Nb₃O₁₀, HPb₂Nb₃O₁₀, PbBi₂Nb₂O₉, BiVO₄, BiCu₂VO₆, BiSn₂VO₆, SnNb₂O₆, AgNbO₃, AgVO₃, AgLi_(1/3)Ti_(2/3)O₂, AgLi_(1/3)Sn_(2/3)O₂, WO₃, BaBi_(1−x)In_(x)O₃, BaZr_(1−x)Sn_(x)O₃, BaZr_(1−x)Ge_(x)O₃, and BaZr_(1−x)si_(x)O₃; acid oxynitrides such as LaTiO₂N, Ca_(0.25)La_(0.75)TiO_(2.25)N_(0.75), TaON, CaNbO₂N, BaNbO₂N, CaTaO₂N, SrTaO₂N, BaTaO₂N, LaTaO₂N, Y₂Ta₂O₅N₂, (Ga_(1−x)Zn_(x))(N_(1−x)O_(x)) (Zn_(1+x)Ge)(N₂O_(x)) (where x represents a value of 0 to 1), and TiN_(x)O_(y)F_(z); nitrides such as NbN and Ta₃N₅; sulfides such as CdS; selenides such as CdSe; L^(x) ₂Ti₂S₂O₅ (L^(x): Pr, Nd, Sm, Gd, Tb, Dy, Ho, or Er); and oxysulfide compounds including La and In (Chemistry Letters, 2007, 36, 854-855); however, the material is not limited to the materials listed here as examples.

The method for forming the first photocatalyst layer 16 is not particularly limited, and examples thereof include a vapor deposition method (for example, a chemical vapor deposition method, a sputtering method, a pulse laser deposition method), and a particle transfer method.

The first photocatalyst layer 16 may have a co-catalyst carried and supported on its surface. In a case where the co-catalyst is carried and supported, the onset potential and the gas generation efficiency become satisfactory. Specific examples of the co-catalyst are as described above.

A method for carrying and supporting the co-catalyst is not particularly limited, and examples thereof include an immersion method (for example, a method of immersing the photocatalyst layer in a suspension containing the co-catalyst) and a vapor deposition method (for example, a sputtering method).

<Cathode electrode>

In the cathode electrode 20, a conductive layer 24, a p-type semiconductor layer 26, an n-type semiconductor layer 28, and a co-catalyst 32 are laminated in this order on a surface 22 a of an insulating substrate 22. In the example of FIG. 1, a semiconductor layer 29 is constituted of the p-type semiconductor layer 26 and the n-type semiconductor layer 28.

(Insulating substrate)

The insulating substrate 22 is a substrate that supports the conductive layer 24 and the semiconductor layer 29 and is constituted of a material having electrical insulation.

The insulating substrate 22 is not particularly limited. For example, a soda lime glass substrate (hereinafter, referred to as an SLG substrate) or a ceramic substrate can be used.

In addition, as the insulating substrate 22, a metal substrate on which an insulating layer is formed may be used.

Further, as the insulating substrate 22, a glass plate such as high strain point glass and non-alkali glass, or a polyimide material can be used.

The insulating substrate 22 may or may not be flexible.

The thickness of the insulating substrate 22 is not particularly limited and may be, for example, about 20 to 20,000 μm, preferably 100 to 10,000 μm, and more preferably 1,000 to 5,000 μm.

(Conductive layer)

The conductive layer 24 is formed on the surface 22 a of the insulating substrate 22 and is used, for example, in order to apply a voltage to the semiconductor layer 29.

The conductive layer 24 is not particularly limited as long as it has conductivity. For example, the conductive layer 24 is constituted of a metal such as Mo, Cr, and W, or a combination thereof. Among these, the conductive layer 24 is preferably constituted of Mo.

The conductive layer 24 may have a single-layer structure or a laminate structure such as a two-layer structure.

A thickness of the conductive layer 24 is generally about 800 nm, but the conductive layer 24 preferably has a thickness of 400 nm to 1 μm.

(Semiconductor layer)

The semiconductor layer 29 generates an electromotive force. The semiconductor layer 29 has the p-type semiconductor layer 26 formed on a surface 24 a of the conductive layer 24 and the n-type semiconductor layer 28 formed on a surface 26 a of the p-type semiconductor layer 26, and a pn junction is formed at the interface between the p-type semiconductor layer 26 and the n-type semiconductor layer 28.

Light incident on the semiconductor layer 29 is absorbed in the semiconductor layer 29 and excites electrons from the valence band of the semiconductor to the conduction band. Then, among the carriers excited by the internal electric field created by the pn junction, electrons are moved to the n-type semiconductor side and positive holes are moved to the p-type semiconductor side.

In the semiconductor layer 29, the conduction types of the p-type semiconductor layer 26 and the n-type semiconductor layer 28 can be measured by a measuring device (product name “PN-12α”, manufactured by NAPSON Corporation) using the Seebeck effect.

The p-type semiconductor layer 26 is constituted of a CIGS compound semiconductor containing Cu, In, Ga, and Se. Specifically, the CIGS compound semiconductor containing Cu, In, Ga, and Se may be a compound represented by Cu(In, Ga)Se₂, or may be a compound represented by Cu(In, Ga)(Se, S)₂ in which a part of Se in Cu(In, Ga)Se₂ is substituted with S.

A molar ratio of Ga (Ga ratio) to a total molar amount of Ga and In in the CIGS compound semiconductor constituting the p-type semiconductor layer 26 is 0.4 to 0.8.

The Ga ratio is 0.4 or more, preferably 0.45 or more, and more preferably 0.5 or more. In the above-described ratio, the conduction band edge of the p-type semiconductor layer 26 becomes shallower, the band discontinuity (band barrier) with the n-type semiconductor layer 28 is eliminated, and thus electrons can easily flow into the n-type semiconductor layer 28.

The higher the Ga ratio is, the better the onset potential is. On the other hand, in a case where the Ga ratio is too high, some problems may occur including the problems that the band gap may be widened, that the increase in the melting point of the CIGS compound semiconductor hinders the grain growth and reduces the grain size, and that the crystallinity of the CIGS compound semiconductor is reduced. In particular, in a case where the Ga ratio exceeds 0.7, these problems become remarkable, and thus the current value decreases. For this reason, the Ga ratio is preferably 0.7 or less, more preferably 0.65 or less, and still more preferably 0.6 or less.

In the present invention, the Ga ratio is calculated based on elemental analysis of the entire semiconductor layer (CIGS compound semiconductor) using high-frequency inductively coupled plasma emission spectrometric analysis method (ICP-AES).

A method for forming the p-type semiconductor layer 26 (CIGS compound semiconductor) includes, for example, a multi-source vapor deposition method (preferably a three-step method), a selenization method, a sputtering method, a hybrid sputtering method, a mechanochemical process method, a screen printing method, a proximity sublimation method, Metal Organic Chemical Vapor Deposition (MOCVD) method, and a spray method (wet film forming method), and among these, a multi-source vapor deposition method is preferable, and a three-step method is more preferable.

The thickness of the p-type semiconductor layer 26 is preferably 0.5 to 3.0 μm and more preferably 1.0 to 2.0 μm.

The n-type semiconductor layer 28 forms the pn junction at the interface with the p-type semiconductor layer 26 as described above. Further, the n-type semiconductor layer 28 is preferably a layer through which the light L is transmitted so that the incident light L reaches the p-type semiconductor layer 26.

Examples of the material constituting the n-type semiconductor layer 28 include a metal sulfide containing at least one metal element selected from the group consisting of Cd, Zn, Sn, and In. The metal sulfide may be used singly or in combinations of two or more thereof.

Examples of the metal sulfide include CdS, ZnS, Zn(S, O), Zn(S, O, OH), In₂S₃, SnS, and SnS_(x)Se_(1−x) (X represents a numerical value of 0 or more and less than 1), CdS and ZnS are preferable, and CdS is more preferable. In particular, CdS is preferable from the viewpoint of lattice matching with the CIGS compound semiconductor. In this case, a film can be formed on the CIGS compound semiconductor in a state where CdS is lattice-matched (epitaxial), and defects at the junction interface can be reduced.

The thickness of the n-type semiconductor layer 28 is preferably 10 nm to 2 μm, and more preferably 15 to 200 nm.

For forming the n-type semiconductor layer 28, for example, a chemical bath deposition method (hereinafter, referred to as a CBD method) is used.

In addition, a window layer may be provided on the n-type semiconductor layer 28. This window layer is constituted of, for example, a ZnO layer having a thickness of about 10 nm.

(Co-catalyst)

The co-catalyst 32 is formed on the semiconductor layer 29, that is, on the surface 28 a of the n-type semiconductor layer 28. The co-catalyst 32 can further improve the onset potential and the photocurrent density of the cathode electrode 20.

The co-catalyst 32 may be formed on the entire surface of the n-type semiconductor layer 28 or may be formed to be scattered in an island shape.

As a material constituting the co-catalyst 32, for example, a simple substance constituted of Pt, Pd, Ni, Au, Ag, Ru, Cu, Co, Rh, Ir, Mn, or the like, an alloy in which the simple substances are combined, and an oxide of the simple substance. Among these, Pt, Rh, or Ru is preferable from the viewpoint that the effects of the present invention are more exhibited.

The size of the co-catalyst 32 is not particularly limited and is 0.5 nm to 1 μm, and the height is preferably about several nm.

The co-catalyst 32 can be formed by, for example, a coating baking method, a photoelectric deposition method, a vacuum vapor deposition method, a sputtering method, an impregnation method, or the like.

(Other members that can be included in cathode electrode)

Cathode electrode 20 may have a metal layer (not shown in Figures) between the n-type semiconductor layer 28 and the co-catalyst 32. In this case, the co-catalyst 32 is formed on the surface of the metal layer.

The metal layer can impart conductivity to a surface layer of the n-type semiconductor layer 28. Therefore, carriers (electrons) generated in the semiconductor layer 29 can easily move on the side of the co-catalyst 32 by the metal layer.

The metal layer is preferably constituted of a transition metal of Group IV or higher. Examples of transition metals of Group IV or higher include Ti, Zr, Mo, Ta, and W.

The thickness of the metal layer is preferably 8 nm or less and more preferably 6 nm or less. The lower limit of the metal layer is not particularly limited as long as the above-mentioned function can be satisfactorily exhibited and the metal layer can be produced.

The metal layer can be formed by, for example, a sputtering method, a vacuum vapor deposition method, an electron beam vapor deposition method, or the like.

The cathode electrode 20 may have another layer other than the above layers. As another layer, for example, a surface protection layer that can be formed on the co-catalyst 32 can be mentioned.

In FIG. 1, an aspect in which the cathode electrode 20 has the insulating substrate 22 is described as an example. However, as long as the effects of the present invention can be exhibited, any one of these members may not be included.

<Diaphragm>

The diaphragm 30 is disposed between the anode electrode 10 and the cathode electrode 20 so that ions included in the electrolytic aqueous solution S can freely enter and exit the anode electrode chamber 42 and the cathode electrode chamber 44, but the gas generated at the anode electrode 10 and the gas generated at the cathode electrode 20 do not mix.

A material constituting the diaphragm 30 is not particularly limited and includes a known ion exchange membrane.

In FIG. 1, an example in which the diaphragm 30 is provided. However, the present invention is not limited thereto, and the diaphragm 30 may not be provided.

<Other configurations>

The gas generated at the anode electrode 10 can be collected from a pipe (not shown in Figures) connected to the anode electrode chamber 42. The gas generated at the cathode electrode 20 can be collected from a pipe (not shown in Figures) connected to the cathode electrode chamber 44.

Although not shown in Figure, a supply pipe, a pump, and the like for supplying the electrolytic aqueous solution S may be connected to the bath 40.

FIG. 1 shows an example in which the inside of the bath 40 is filled with the electrolytic aqueous solution S; however, the present invention is not limited to this, and the inside of the bath 40 may be filled with the electrolytic aqueous solution S at the time of driving the water splitting device.

FIG. 1 shows the case in which both the anode electrode 10 and the cathode electrode 20 are photocatalytic electrodes; however, the present invention is not limited to this, and only the cathode electrode 20 may be a photocatalytic electrode.

FIG. 1 shows an example in which the anode electrode 10, the diaphragm 30, and the cathode electrode 20 are arranged in this order along the direction intersecting the traveling direction of the light L. However, the present invention is not limited to this, and the water splitting device of the embodiment of the present invention may have the structure shown in FIG. 2.

FIG. 2 is a lateral view schematically illustrating a water splitting device 100 that is one embodiment of the water splitting device of the embodiment of the present invention. The water splitting device 100 is a device that generates gases from the anode electrode 110 and the cathode electrode 120 by irradiation with light L. Specifically, water is decomposed by the light L, oxygen is generated from the anode electrode 110, and hydrogen is generated from the cathode electrode 120.

As shown in FIG. 2, the water splitting device 100 includes: a bath 40 filled with the electrolytic aqueous solution S; the anode electrode 110 and the cathode electrode 120 each disposed inside the bath 40; and a diaphragm 30 disposed between the anode electrode 110 and the cathode electrode 120 and inside the bath 40. The anode electrode 110, the diaphragm 30, and the cathode electrode 120 are arranged in this order along the traveling direction of the light L. Since the water splitting device 100 is the same as the water splitting device 1 in FIG. 1, except that the disposition of the anode electrode 110, the disposition of the cathode electrode 120, and the direction of irradiation with the light L are different from the water splitting device 1, the different points will be mainly described.

The anode electrode 110 is disposed in the bath 40 so that the first photocatalyst layer 116, the first conductive layer 114, and the first substrate 112 are arranged in this order from the side irradiated with the light L.

In the cathode electrode 120, a co-catalyst 132, an n-type semiconductor layer 128, a p-type semiconductor layer 126, a conductive layer 124, and an insulating substrate 122 are arranged in the bath 40 in this order from the side irradiated with the light L. The semiconductor layer 129 is constituted of the p-type semiconductor layer 126 and the n-type semiconductor layer 128.

The anode electrode 110 and the cathode electrode 120 are disposed to be inclined so that the amount of incident light per unit area increases.

In the water splitting device 100, the first substrate 112 and the first conductive layer 114 are preferably transparent in order to make the light L incident on the cathode electrode 120. As a result, the light that can not be absorbed by the first photocatalyst layer 116 can be used by the cathode electrode 120, and thus there is an advantage that the light use efficiency per unit area is improved.

The term “transparent” in the present invention means that the light transmittance in the wavelength range of 380 nm to 780 nm is 60% or higher. The light transmittance is measured using a spectrophotometer. As the spectrophotometer, for example, V-770 (product name) manufactured by JASCO Corporation, which is an ultraviolet-visible spectrophotometer, is used.

The photocatalytic electrode for water splitting according to the embodiment of the present invention includes a semiconductor layer containing a CIGS compound semiconductor containing Cu, In, Ga, and Se, in which a molar ratio of Ga to a total molar amount of Ga and In in the CIGS compound semiconductor is 0.4 to 0.8. The details of the photocatalytic electrode for water splitting according to the embodiment of the present invention are the same as those described for the cathode electrode 20 in the water splitting device 1, and thus the description thereof is omitted.

Second Embodiment

In one embodiment of a water splitting device of the present invention, the water splitting device generates gases from a photocatalytic electrode for hydrogen generation and a photocatalytic electrode for oxygen generation by irradiating the photocatalytic electrode for hydrogen generation and the photocatalytic electrode for oxygen generation with light, and includes a bath to be filled with an electrolytic aqueous solution and the photocatalytic electrode for hydrogen generation and the photocatalytic electrode for oxygen generation each disposed in the bath, in which the photocatalytic electrode for hydrogen generation has a p-type semiconductor layer, an n-type semiconductor layer provided on the p-type semiconductor layer, and a co-catalyst provided on the n-type semiconductor layer, and a band offset ΔE which is a difference between a potential p-CBM at a lower end of a conduction band of the p-type semiconductor layer and a potential n-CBM at a lower end of a conduction band of the n-type semiconductor layer satisfies the following relationship.

ΔE=(n-CBM)−(p-CBM)≤0.1 [eV]

The photocatalytic electrode (specifically, the photocatalytic electrode for hydrogen generation) in the water splitting device of the second embodiment shows an excellent onset potential. Although the details of the reasons for this property have not been clarified, it is assumed that the reasons are generally as follows.

That is, in a case where ΔE is 0.1 eV or less, carrier transport is difficult to be inhibited. This is probably a reason for the fact that the photocatalytic electrode in the water splitting device of the second embodiment exhibits an excellent onset potential.

In the second embodiment, ΔE is 0.1 eV or less and preferably 0 eV or less from the viewpoint that the onset potential of the photocatalytic electrode is more excellent. In addition, the lower limit of ΔE is preferably −0.5 eV or more.

The p-type semiconductor layer in the second embodiment is not particularly limited as long as ΔE can be 0.1 eV or less, but it is preferable to use the p-type semiconductor layer described in the first embodiment since it is easy to set ΔE to 0.1 eV or less.

The n-type semiconductor layer in the second embodiment is not particularly limited as long as ΔE can be 0.1 eV or less, but it is preferable to use the n-type semiconductor layer described in the first embodiment since it is easy to set ΔE to 0.1 eV or less.

In the water splitting device of the second embodiment, the configuration other than the p-type semiconductor layer and the n-type semiconductor layer is the same as that of the water splitting device of the first embodiment, and the description thereof is omitted.

EXAMPLES

Hereinafter, the present invention will be described in more detail with reference to Examples. Materials, used amounts, ratios, treatments, treatment procedures, and the like described in Examples can be appropriately changed without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be restrictively interpreted by the following Examples.

Example 1

A photocatalytic electrode of Example 1 was produced as follows.

First, a layer (conductive layer) having a thickness of 600 nm and formed of Mo was formed on the surface of a soda-lime glass substrate (insulating substrate) by a direct current (DC) magnetron sputtering method using a magnetron sputtering apparatus (product name “CFS-12P”, manufactured by Shibaura Eletech Corporation).

Next, a layer (p-type semiconductor layer) formed of a CIGS compound semiconductor having a thickness of 2 μm was formed on the surface of the conductive layer by a three-step method using a multi-source vapor deposition apparatus (product name “EW-10”, manufactured by EIKO Engineering Corporation). Specifically, Cu, In, Ga, and Se were used as vapor deposition sources, the substrate temperature of the first step was set to 400° C., and the substrate temperatures of the second and third steps were set to 520° C. The Ga ratio (Ga/(Ga+In)) was adjusted by controlling the vapor deposition rates of Ga and In.

Next, a layer (n-type semiconductor layer) formed of CdS having a thickness of 70 nm was formed on the surface of the p-type semiconductor layer by a chemical bath deposition method (CBD method) using 1.5 mM CdSO₄, 1.5 M NH₄OH, and 7.5 mM thiourea aqueous solution. The CBD method was performed at 70° C. for 40 minutes.

Next, Pt particles (co-catalyst) having a size of 0.5 nm were carried and supported on the surface of the n-type semiconductor layer by a DC magnetron sputtering method using a magnetron sputtering apparatus (product name “MSP-30T”, manufactured by Vacuum Devices Inc.).

In this manner, a photocatalytic electrode (photocatalytic electrode for hydrogen generation) of Example 1 having the same structure as the cathode electrode 20 of FIG. 1 was obtained.

Examples 2 to 5, Comparative Example 1

In the case of forming the p-type semiconductor layer, photocatalytic electrodes (photocatalytic electrodes for hydrogen generation) of Examples 2 to 5 and Comparative example 1 were obtained in the same manner as in the production of the photocatalytic electrode of Example 1, except that the vapor deposition rates of Ga and In were appropriately changed so that the Ga ratio became a value shown in Table 1.

[Measurement of Ga ratio]

Using an apparatus (product name “ICP-AES8100”, manufactured by Shimadzu Corporation) based on high-frequency inductively coupled plasma emission spectrometric analysis method (ICP-AES) as a measurement principle, the entire elements of the p-type semiconductor layer (CIGS compound semiconductor) were analyzed, the molar amounts of Ga and In were calculated, and the Ga ratio (Ga/(Ga+In)) was calculated from the obtained molar amounts. Specifically, the CIGS compound semiconductor contained in the p-type semiconductor layer was dissolved in an appropriate dissolving solution (hydrofluoric acid), and quantitative measurement was performed. The results are shown in Table 1.

[Evaluation test]

<Onset potential measurement>

The onset potential of the produced photocatalytic electrode was evaluated by an electrochemical measurement in a three-electrode system using a potentiostat (product name “HZ-7000”, manufactured by Hokuto Denko Corporation).

Specifically, an electrochemical cell made of Pyrex glass was used, each photocatalytic electrode of the above Examples and Comparative examples were used as a working electrode, an Ag/AgCl electrode was used as a reference electrode, and a Pt wire was used as a counter electrode. As the electrolytic aqueous solution, an aqueous solution containing 0.5 M sodium sulfate, 0.25 M sodium dihydrogen phosphate; and 0.25 M disodium hydrogen phosphate was used.

The inside of the electrochemical cell was filled with argon, and dissolved oxygen and carbon dioxide were removed by performing sufficient bubbling before the measurement.

As a light source, a solar simulator (AM1.5G) (product name “XES-70S1” manufactured by SAN-EI ELECTRIC CO.) was used.

Then, the voltage was swept from 0 V (vs. RHE) to 0.8 V (vs. RHE) at 20 mV/s to obtain a current-potential curve under the simulated sunlight irradiation by the solar simulator.

Based on the value of the obtained onset potential, the evaluation was performed according to the following evaluation criteria. The evaluation results are shown in Table 1.

A: Onset potential value is 0.6 V (vs. RHE) or more.

B: Onset potential value is less than 0.6 V (vs. RHE).

<Photocurrent density>

The photocurrent density (mA/cm²) at 0.6 V (vs. RHE) was read from the current-voltage curve obtained by the above-mentioned onset potential measurement and evaluated according to the following evaluation criteria. The evaluation results are shown in Table 1.

A: Photocurrent density is 4 mA/cm² or more.

B: Photocurrent density is more than 0 mA/cm² and less than 4 mA/cm².

C: Photocurrent density is 0 mA/cm² or less.

<Band offset ΔE>

The potential at the upper end of the valence band of the p-type semiconductor layer of the produced photocatalytic electrode was determined using an atmospheric photoelectron spectrometer (product name “AC-3”, manufactured by Riken Keiki Co., Ltd.). The band gap value of the p-type semiconductor layer of the produced photocatalytic electrode was determined using an ultraviolet-visible spectrophotometer (product name “V-770”, manufactured by JASCO Corporation). The p-CBM of the p-type semiconductor layer in the photocatalytic electrode was determined by adding the value of the band gap to the above-described obtained potential at the upper end of the valence band of the p-type semiconductor layer.

In addition, the n-CBM of the n-type semiconductor layer in the photocatalytic electrode was determined in the same manner as in the measurement of p-CBM, except that the n-type semiconductor layer was used.

ΔE was determined from the obtained values of the p-CBM and n-CBM. The results are shown in Table 1.

TABLE 1 Onset Photocurrent Band potential density offset Value Value ΔE Ga [V (vs. Evaluation [mA/ Evaluation Value ratio RHE)] result cm²] result [eV] Example 1 0.4 0.70 A 2 B 0.08 Example 2 0.5 0.72 A 4 A 0 Example 3 0.6 0.74 A 5 A −0.08 Example 4 0.7 0.74 A 3 B −0.16 Example 5 0.8 0.74 A 2 B −0.24 Comparative 0.3 0.58 B 0 C 0.16 Example 1

As shown in Table 1, it has been confirmed that the use of a photocatalytic electrode having a semiconductor layer containing a CIGS compound semiconductor having a Ga ratio of 0.4 to 0.8 provides an excellent onset potential (Examples 1 to 5).

In addition, as shown in Table 1, it has been confirmed that the use of a photocatalytic electrode having ΔE of 0.1 eV or less provides an excellent onset potential (Examples 1 to 5).

In addition, from the comparison among Examples 1 to 5, it has been confirmed that the use of a photocatalytic electrode having a semiconductor layer containing a CIGS compound semiconductor having a Ga ratio of 0.5 to 0.6 provides an excellent photocurrent density (Examples 2 and 3).

On the other hand, as shown in Table 1, it has been confirmed that the use of a photocatalytic electrode having a semiconductor layer containing a CIGS compound semiconductor having a Ga ratio less than 0.4 provides an inferior onset potential (Comparative example 1).

EXPLANATION OF REFERENCES

1, 100: Device

10, 110: Anode electrode

12, 112: First substrate

14, 114: First conductive layer

16, 116: First photocatalyst layer

20, 120: Cathode electrode

22, 122: Insulating substrate

22 a, 24 a, 26 a, 28 a: Surface

24, 124: Conductive layer

26, 126: p-type semiconductor layer

28, 128: n-type semiconductor layer

29, 129: Semiconductor layer

30: Diaphragm

32, 132: Co-catalyst

40: Bath

42: Anode electrode chamber

44: Cathode electrode chamber

50: Conducting wire

S: Electrolytic aqueous solution

L: light 

What is claimed is:
 1. A water splitting device which generates gases from a photocatalytic electrode for hydrogen generation and a photocatalytic electrode for oxygen generation by irradiating the photocatalytic electrode for hydrogen generation and the photocatalytic electrode for oxygen generation with light, the water splitting device comprising: a bath to be filled with an electrolytic aqueous solution; and the photocatalytic electrode for hydrogen generation and the photocatalytic electrode for oxygen generation each disposed in the bath, wherein the photocatalytic electrode for hydrogen generation has a p-type semiconductor layer, an n-type semiconductor layer provided on the p-type semiconductor layer, and a co-catalyst provided on the n-type semiconductor layer, the p-type semiconductor layer is a semiconductor layer containing a CIGS compound semiconductor containing Cu, In, Ga, and Se, and a molar ratio of Ga to a total molar amount of Ga and In in the CIGS compound semiconductor is 0.4 to 0.8.
 2. The water splitting device according to claim 1, wherein the n-type semiconductor layer contains CdS.
 3. The water splitting device according to claim 1, further comprising: a metal layer provided between the n-type semiconductor layer and the co-catalyst.
 4. The water splitting device according to claim 1, wherein the molar ratio of Ga to the total molar amount of Ga and In in the CIGS compound semiconductor is 0.5 to 0.7.
 5. A water splitting device which generates gases from a photocatalytic electrode for hydrogen generation and a photocatalytic electrode for oxygen generation by irradiating the photocatalytic electrode for hydrogen generation and the photocatalytic electrode for oxygen generation with light, the water splitting device comprising: a bath to be filled with an electrolytic aqueous solution; and the photocatalytic electrode for hydrogen generation and the photocatalytic electrode for oxygen generation each disposed in the bath, wherein the photocatalytic electrode for hydrogen generation has a p-type semiconductor layer, an n-type semiconductor layer provided on the p-type semiconductor layer, and a co-catalyst provided on the n-type semiconductor layer, and a band offset ΔE which is a difference between a potential p-CBM at a lower end of a conduction band of the p-type semiconductor layer and a potential n-CBM at a lower end of a conduction band of the n-type semiconductor layer satisfies the following relationship, ΔE=(n-CBM)−(p-CBM)≤0.1 [eV].
 6. A photocatalytic electrode for water splitting, the electrode comprising: a semiconductor layer containing a CIGS compound semiconductor containing Cu, In, Ga, and Se, wherein a molar ratio of Ga to a total molar amount of Ga and In in the CIGS compound semiconductor is 0.4 to 0.8.
 7. The water splitting device according to claim 2, further comprising: a metal layer provided between the n-type semiconductor layer and the co-catalyst.
 8. The water splitting device according to claim 2, wherein the molar ratio of Ga to the total molar amount of Ga and In in the CIGS compound semiconductor is 0.5 to 0.7.
 9. The water splitting device according to claim 3, wherein the molar ratio of Ga to the total molar amount of Ga and In in the CIGS compound semiconductor is 0.5 to 0.7. 